Electric control system



Feb. 23, 1937. H. E. YOUNG ELECTRIC CONTROL SYSTEM Filed June 20, 1932'2 Sheets-Sheet 1 Jfiaera 07 jfu you 47 +7 Feb. 23, 1937. H. E. YOUNGELECTRIC CONTROL SYSTEM Filed June 20, 1932 2 Sheets-Sheet 2- I youPatented Feb. 23, 1937 UNlTED STATES PATENTOFFICE ELECTRIC CONTROLSYSTEM Hugh E. Young, Chicago, Ill.

Application June 20, 1932, Serial No. 618,369

15 Claim.

This invention provides a new system and method for controlling theeffective applied voltage from a source of alternating current to keepthe elective flow of current per unit of time through a loadsubstantially constant, even though the eifective resistance of the loadto current flow may vary from substantially short circuit value tomaximum load, i. e., maximum efl'ective resistance for which the systemis designed.

The principal feature of my invention is the provision of means formaintaining the current substantially constant regardless of variationsin the line load. My invention is particularly useful in connection withstreet lighting systems; wherein a relatively large number of lamps areconnected in series. My invention is not to be limited to a lamp load,but is applicable equally to any situation wherein the load changeseither by cutting in or out or otherwise changing resistance orreactance or counter-electromotive force, or any other known oppositionto current flow.

According to my invention, the applied voltage is initially made greatenough to force the desired current through the resistance or oppositionof the load when it is greatest. Then, as the resistance or oppositionto current flow becomes or is made less, the current tends to rise,

but is effectively limited through the employment of a tube, the firingtime of which per half cycle is so controlled by the current flowproduced as to select the desired voltage and its period of applicationas will keep the efiective current flow per unit of time through theload substantially constant for all variations within the design of thesystem.

In the preferred practice of my invention, the tube is a grid controlledarc rectifying tube disposed between the anode transformer and the loadcircuit in which predetermined current flow is to be maintained. By theemployment of a current sensitive regulator, the phase of gridexcitation is made shiftable through substan- 5 tially 180 electricaldegrees as the effective resistance or opposition to current flow shiftsfrom maximum toward minimum. Thus, no matter what the effectiveresistance or opposition to current may be, the tube suppliesautomatically 50 thenecessary or proper part of each half cycle to keepthe eflectlve current flow per unit of time substantially constant.

As a modification, the tube, instead of being directly between thealternating current line and 56 the load, may be disposed in a controlcircuit,

the current through which is so regulated and applied to a saturablecore reactor in the load circuit as to keep the current in the loadcircuit constant over a like range of load resistance variation, 1. e.,from maximum to substantially short circuit value.

Other features and advantages of my invention will appear more fully asI proceed with my specification.

In those forms of device embodying the fea- 10 tures of my inventionshown in the accompanying drawings- Figure 1 is a diagrammatic viewshowing my invention employed in a system with direct currentdistribution; and Fig. 2 is a modified view 15 showing the applicationof the invention to a system with alternating current distribution.

It is to be understood that all the drawings are diagrammatic views, inwhich no attempt is made to show the necessary and usual fuses,cut-outs, relays, protective devices, and the like, commonly used inelectrical systems of the character herein described.

In the device of Fig. 1, III, III indicate the A. C. supply, II the loadcircuit with lights I2, I2 connected in series. The lights I2, I2 may beeither of the incandescent, glow or the arc discharge type, theincandescent type being preferred. It 'is to be understood, of course,that in connection with each light, there is provided the customaryautomatic cut-out or short circuiter (not shown) to prevent interruptionto the line flow when the light burns out or is turned out. It will beseen that with the lights connected in series, as shown, the resistance,and consequently the ,load of the line will vary, depending upon thenumber of lights in use.

In a system of the kind just described, I provide means for maintainingthe current substantially constant regardless of variations In the lineload. I shall now describe the construction and operation of such means.I3, I3 indicate rectifying tubes of the three-element type well known inthe electrical arts. I3", I3 indicate the filaments, I3", I3 the gridsand I3, I3 the anodes of tubes I3, I3. I4 indicates a transformer forsupplying current to heat the filaments I3 I3. I 5 indicates a gridtransformer for supplying voltage to the grids I3, I3 One end of primarywinding P of grid transformer I5 is connected to line II by line I1, theother end of this winding being connected by line I8 to the commonconnection of resistance 23 and coil of solenoid I3. The secondary S ofgrid transformer I5 has its ends connected to grids I3", I3" by linesIt, I3, while the center tap of this winding is connected to line I I byline 20. 29 indicates the anode transformer which supplies voltage toanodes l3, |3 of tubes I3, I3. The primary P of anode transformer 29connects to the supply lines HI, ID. The secondary S of anodetransformer 23 has its ends connected to anodes l3", |3 of tubes I3, I3, while its center tap connects to load circuit through the coil of D.C. solenoid 28. Intermediate taps Tl, T2 are connected to resistance 23,and coil of solenoid l9, respectively. l9 indicates the A. C. solenoidwith an iron plunger l9, constituting a variable inductance. 20indicates a series D. C. solenoid with an iron plunger 20. The plungersl9 and 20 are connected to opposite ends of lever 2| which is pivoted,as shown. 22 indicates a spring, and 22 a thumb nut for adjustingtension of spring 22 so as to obtain an electrical balance of lever 2|.23 indicates a resistance, one end of which is connected to tap T, ofsecondary S of anode transformer 29, and the other end to coil ofsolenoid I9.

In the apparatus of Fig. 1 the tubes l3, l3 are of the rectifying typewith grid control. Control is effected by applying A. C. voltage to boththe anodes, l3, l3 and. the grids I3 Q3 The aver age current flowing canbe controlled by varying the grid voltage with respect to the anodevoltage, for example, as here shown, this is accomplished by varying thephase of the grid voltage with respect to the phase of the anodevoltage. For example, as here shown, this varying is obtained splittingthe phase. In the operation of the ap paratus, as the line load orresistance decreases (for example, by burning out and short-circuit-.ing of one or more lights l2) the current tends to increase, and asthis load current flows through the D. C. series coil of solenoid 2B,the increased magnetic pull on plunger 2s pulls this plunger furtherupward into the coil, and through pivoted lever 2|, the plunger it of A.C. solenoid i9 is pushed further down into the coil, and this additionaliron within the coil increases its inductance. The transformer l5reverses the polarity of the split phase so that the grid voltagepolarity will be the same as the plate voltage polarity. This increasein inductance shifts or splits the phase of the grid voltage withrespect to the anode voltage in a direction (more out of phase) whichprevents an undue increase of load current. Should the load current tendto decrease, from any cause, the decreased magnetic pull onplunger 2|)of solenoid 20 will permit spring 22 to pull it down and through actionof lever 2|, plunger ill of solenoid F9 will be pulled upward. Thisremoval of iron from within the coil I9 reduces its inductance, and thischange in inductance shifts the phase of the grid voltage with respectto the anode voltage in a direction (more in phase) which prevents anundue decrease of load current. The load current is thus heldsubstantially constant under all conditions.

Due to the fact that no appreciable current flow is required in the gridexcitation circuit, the mas of the moving core l9 and the distance it isrequired to travel .may be made quite small. The solenoid 20 may be assensitive as desired. Hence, with the present regulator, the phase ofgrid excitation may be changed with such rapidity that as soon as themagnetic effect upon the solenoid core 20* is changed, even within thehalf cycle, the regulator promptly shifts the phase angle of excitationto avoid the injurious effect of excessive current flow for any greatertime than the completion of a half cycle which has already begun to flowdue toflring of the tube.

The regulator of my invention has in connection with the present form ofphase shifting circuit a marked advantage not found in prior devices. Itis to be observed that the regulator comprises a magnet such as asolenoid having the movable member, in this case the core 20 anddirectly connected thereto by the lever 2|, a movable part of areactance IS. The movable part in this case is the core IS. Thereactance part l9 creates by its movement a voltage in the reactancebranch T This voltage is independent of the increase or decrease duesolely to its change of position, and is something additional caused bythe movement itself. The movement of the core l9 in the coil induces avoltage which affects the grid excitation. As a result, the response issharper and more rapid than it is possible to secure in known devices.The small inertia of the parts of the regulator and the consequent rapidmovement due to variations in current in the direct current circuit aidsin instantaneous response. Tests of my system show a uniformity orconstancy of effective current flow in the lamp circuit such that with asensitive ammeter in series with the lamps, no detectable variationoccurs as the lamps are successively all switched i or out of the seriescircuit.

The device of Fig. 2 shows the invention applied to apparatus in whichA. C. power is delivered to the line Hi. In this apparatus, I use asaturable core reactor I26 the A. C. coil i26 of which is in. serieswith the solenoid E20 and the load i H. I then employ rectifying tubessimilar to the rectifying tubes i3, 33 to supply D. C. to the D. C. coilof the reactor. The rectifying tubes are provided with control grids andthe grid voltage with respect to the anode voltage is varied byvariations in the line current in a manner similar, to the way in whichthe grids of the tubes l3, l3 are controlled. The result of this is tovary the flow of D. C. through the D. C. coil of the reactor, thusvarying the inductance of the A. C. coil thereof. The apparatus is soarranged that a tendency for the line load to increase will increase theinductance, and vice versa. Thus, the line current is maintainedsubstantially constant, even though the load varies.

In the device of Fig. 2, the wires H0, H indicate the A. C. supply.indicates the A. C. series load circuit which is in series with coil H6of reactor I26, the load thereon comprising the series connected lightsH2, 2 which may be either of the incandescent, glow or are dischargetype.

In a system of the kind just described, I provide means for maintainingthe current substantially constant regardless of variations in the lineload. I shall now describe the construction and operation of such means.H3, 3 indicate the rectifying tubes which are of the three-element type.H3 3 indicate the filaments, H3", 3 the grids and 3, ||3 the anodes oftubes I [3, H3. H4 indicates the transformer for supplying current toheat the filaments H3, H3 of tubes H3, H3. indicates the gridtransformer for supplying voltage with reverse polarity to the grids H33 of tubes H3, H3. One end of primary winding P of grid transformer H5is connected to line I21 by line H1, and the other end of-this windingis connected by line I H 8 to the common connection of resistance I23,and coil of solenoid H9. The secondary S of grid transformer H5 has itsends connected to balance of lever III.

grids Ill, lil'byline Ill, III, while the centertapofthiswindingisconnectedtoline I" by line I. I" indicates the anodetransformer which supplies voltage to anodes 8, 8 of tubes H8. Theprimary P of anode transformer III connects to the supply lines III,III. The secondary B of anode transformer III has its ends connected toanodes 8, III", while its center tap connects to D. C. circuit II'Ithrough D. C. coil III of reactor ill. Intermediate taps TI, T2 areconnected to resistance I28 and coil at solenoid I II, respectively. ll6 indicates the A. C. solenoid with an iron plunger Ill, constituting avariable inductance. III indicates the A. C. solenoid with iron plungerIII, the winding of which is in series of load circuit Ill and A. C.coil I26 of reactor ill. The plungers Ill and III- are connected toopposite ends of lever III which is pivoted as shown. It! indicates thespring, and If! a thumb nut for adjusting tension of spring If! so as toobtain an electrical ill indicates a resistance of a correct ohmagewhich may be adJustable, one end of which is connected to tap Tl ofsecondary S of anode transformer I", and the other end to coil solenoidIII. III indicates a saturable core reactor with core legs and coils soarranged that there is no transformer action between the two windings.No attempt is made in the diagram of Fig. 2 to show construction detailsof this reactor. Coil I26 is the A. C. winding of reactor I26, and beingin series with line Ill carries the full load of the series circuit.Coil I26 is the D. C. winding of reactor I26 and carries the rectifledfull wave direct current supplied by the rectifying tubes Ill, Ill. CoilI26 of the reactor is connected in series with D. C. circuit III.

In the apparatus of Fig. 2, the tubes Hi, I II are of the rectifyingtype with grid control. Control is efl'ected by applying A. C. voltageto both the anodes 8, III and the grids H3", Ill". The average currentflowing can be controlled by varying the phase of the grid voltage withrespect to the anode voltage. In the operation of the apparatus, ifsumcient direct current is passed through coil I26 of reactor III tosaturate its core, the inductance of A. C. coil I26 will be greatlyreduced and circuit III with its load III will have minimum impedance(resistance to current flow) With no direct current flowing in coil I26of reactor, A. C. coil I26- will act as an iron cored choke and circuitIII with its load III will have its greatest impedance (resistance tocurrent With any intermediate amount of direct current flowing in coilI26", the inductance or choking effect of coil I26- will vary, dependingupon the amount of direct current flowing in coil I26". With alladjustments made and the required current flowing in line iii, if theline load or resistance decreases (for example, by burning out andshort-circuiting of one or more lights I I!) the current tends toincrease in circuit ill, but since the coil of A. C. solenoid I26 is inseries with load circuit l I I, the increased current exerts a strongermagnetic pull on iron plunger III- pulling it upward within the coil.Through the pivoted lever ill, the upward motion of plunger l2. pushesiron plunger ll! of solenoid III further downward within its coll, thusincreasing the inductance of coil 9. This increase of inductance shiftsor splits the phase of the grid voltage with respect to the anodevoltage in a direction (more out of phase) so that less direct currentflows in coil III of reactor I26. The decrease of direct current in coilI26" increases the inductance, or choking eflect of coil I26-suiiiciently to prevent any undue increase in load current. If thecurrent in the load circuit Ill should tend to de-.

crease, from any cause, the decreased magnetic pull on iron plunger illof solenoid ill will cause spring III to pull down this end of pivotedlever III, thus raising plunger HI slightLv out of coil III whichdecreases its inductance. This decrease of inductance shifts or splitsthe phase of the grid voltage with respect to the anode voltage in adirection (more in phase) so that more direct current flows, in D. C.coil I26" of reactor I 26. This increase of direct current decreases theinductance or choking eifect of A. C. coil I26- sumciently to preventany undue decrease in the load current. Thus a substantially constantcurrent will be maintained regardless of variations in the load circuit.

While I have shown a speciflc embodiment in which the load compriseslamps, the opposition of which to current flow is chiefly resistance, itis to be understood that I do not intend to conflne the invention to anyspecific load, nor any speciflc form of resistance to current flow, asobviously the specific form of the load or its particular form ofopposition of current flow may be varied without departure from myinvention.

While I speak herein of constant current, it is not intended, as thoseskilled in the art appreciate, that the fluctuations due to currentimpulses must be smoothed out. I intend the phrase constant current tomean that the eifective current per unit of time is constant or nearenough constant for the purposes intended.

While I have shown and described certain embodiments of my invention, itis to be understood that it is' capable of many modifications. Changes,therefore, in the construction and arrangement may be made withoutdeparting from the spirit and scope of the invention as disclosed in theappended claims, in which it is my intention to claim all noveltyinherent in my invention as broadly as permissible, in view of the priorart.

It is to be noted that since the apparatus of Fig. 1 is designed to givea constant average current, the spring 22 serves to substantiallybalance the pull of the solenoid 20 upon the armature 26' for allpositions of said armature in its range of movement when a predeterminedcurrent is flowing in the solenoid. For example, if it is desired tomaintain the D. C. output substantially constant at say 6 amperes, thespring 22 will substantially balance the pull of the solenoid on thearmature 20' when 6 amperes is flowing through the solenoid coll.Moreover, this balancing will occur at substantially all positions ofthe armature in its range of movement. Consequently, if there is anymomentary change in the current flow, the armature will move until suchpredetermined flow of 6 amperes is again established (through a changein the inductance l9), whereupon the armature will stop in its newposition again balanced by the spring.

What I regard as new, and desire to secure by Letters Patent, is:

l. A system for current regulation comprising an A. C. supply circuit, awork circuit the resistance of which is variable, said work circuitbeing supplied with energy from said supply circuit, a saturable corereactor for controlling the energy supplied through it to said workcircuit, a grid controlled tube having its plate current supplied bysaid supply circuit and having a direct current circuit connected tosaid reactor to saturate the core thereof to variable degree, a gridcontrol circuit for supplying grid excitation for said tube, and meansincluding a phase shift circuit and a regulator in series relation tothe work circuit for shifting the phase of grid excitation to regulatethe flow of current in said work circuit.

2. The system of claim 1 wherein the regulator comprises a magnet havinga movable member and a stationary member and the phase shift circuitincludes a resistance element and a reactance element one of saidelements being adjustable with respect to the other element, and adirect mechanical connection between said movable member and saidadjustable element.

3. In an electrical system of distribution of the class wherein analternating current source energizes a work circuit that is variableover a wide range of load resistance, the combination with said systemof a pair of electric valves inter-, connected between said source andsaid work circuit, said electric valves being connected to conductsuccessive half cycles of the alternating current, a control electrodein each of said valves, phase shift means energized from saidalternating current source for energizing said control electrodes torender said valves conducting at predetermined times in the half cyclesof alternating current to be conducted thereby, and means connected tobe directly responsive to the current flow variation in said loadcircuit for controlling said phase shift means in such manner as tochange the time in each half cycle at which said valves are renderedconducting for maintaining the load current substantially constantregardless of change of said resistance.

4. In an electrical system of distribution of the class wherein analternating current source energizes a work circuit that is variableover a wide range of load resistance, the combination with said systemof a pair of electric valves interconnected between said source and saidwork circuit, said electric valves being connected to conduct successivehalf cycles of the alternating current, a control electrode in each ofsaid valves, a phase shifting circuit comprising a resistor and anadjustable reactor energized from said alternating current source andconnected to said control electrodes to render said valves conducting atpredetermined times in the half cycles of alternating current to beconducted thereby, a winding connected in series circuit relation withsaid 5. An electric power system comprising, in-

combination, a pair of electric valves for connection between a sourceof alternating current and a load circuit requiring substantiallyconstant current the resistance of which is variable over a wide range,said electric valves being connected to conduct successive half cyclesof the alternating current, a control electrode in each of said valves,a phase shifting circuit comprising a resistor and a reactor connectedin series circuit relation for energization from said alternatingcurrent source, said phase shift circuit being connected to said controlelectrodes to render said valves conducting at predetermined times inthe half cycles of alternating current adapted to be conducted thereby,a movable core in said reactor, awinding connected in series circuitrelation with said load-circuit, a movable core in said winding, aspring for biasing said last named movable core against the forceexerted thereon by said winding, and a pivoted arm interconnecting saidcores for moving said first named core in accordance with current flowvariation in said winding to change the inductance of said reactor insuch manner as to change the time in each half cycle at which saidvalves are rendered conducting for maintaining the load currentsubstantially constant regardless of change of said resistance.

6. An electric power system comprising, in combination, a transformerhaving a primary winding for connection to a source of alternatingcurrent and a secondary winding provided with a mid-tap; a pair ofelectric valves each having an anode, a cathode and a control electrode;the anodes of said valves being connected to the terminals of saidsecondary winding and the cathodes being disposed to be commonlyconnected to one terminal of a load circuit requiring substantiallyconstant average current the resistance of which is variable over a widerange, the other terminal of said load circuit being connected to saidmid-tap; a phase shifting circuit comprising a resistor and a reactorconnected in series circuit relation for energization from saidalternating current source, said phase shift circuit being conne'cted tosaid control electrodes to render said valves conducting atpredetermined times in the half cycles of alternating current adapted tobe conducted thereby, a movable core in said reactor, a windingconnected in series circuit relation with said load circuit, a movablecore in said winding, spring means for biasing said last named movablecore against the force exerted thereon by said winding, and a pivotedarm interconnecting said cores for moving said first named core inaccordance with current flow variation in said winding to change theinductance of said reactor in such manner as to change the time in eachhalf cycle at which said valves are rendered conducting for maintainingthe average load current substantially constant regardless of change ofsaid resistance.

'7. In a series lighting system, an alternating current supply system, aload circuit including a plurality of series connected, lamps joined tosaid supply system; a reactor having an alternating current coilconnected in series with said load circuit and a direct current coil, arectifying tube for delivering direct current to said direct currentcoil, and means for causing variations in said load circuit to vary theflow of direct current in said direct current coil by changing the phaseof the grid voltage with respect to the anode voltage of the rectifyingtube.

8. In a series lighting system, an alternating current supply system, aload circuit including a plurality of series connected lamps joined tosaid supply system, a reactor having an alternating current coilconnected in series with said load circuit, a direct current coil onsaid reactor, a rectifying tube for delivering direct current to saiddirect current coil, means for changing the phase of the grid voltagewith respect to the anode voltage of said rectifying tube including agrid voltage circuit with inductance and resistance therein, and meanswhereby variations in the load current vary the inductance.

9. In a series lighting system, an alternating current supply system, aload circuit including load circuit, a direct current coil on saidreactor,

a rectifying tube for delivering direct current to said direct currentcoil, and means for causing variations in said load circuit to vary thephase of the grid voltage with respect to the anode voltage of therectifying tube to vary the flow of direct current in said directcurrent coil, said means including a grid voltage circuit with variableinductance and resistance therein, a solenoid in series with said loadcircuit, and a connection between the solenoid and the inductancewhereby an increase of current in the solenoid tends to increase theinductance, and vice versa.

10. The method of maintaining constant the effective value of current ina load circuit energized from an alternating current source through arectifying tube, the resistance of the load circuit being variable frommaximum to substantially zero value, which comprises firing the tube foreach similar half cycle, and delaying the time of firing within eachhalf cycle as the resistance of the load circuit decreases from themaximum to the zero value in accordance with the increase in currentflow in the load circuit from the constant value.

11. The method for maintaining substantially constant the flow ofcurrent in a load circuit from an alternating current source through agrid controlled rectifying tube, the resistance of which load circuit isvariable from a maximum value to a substantially zero value, whichcomprises applying a firing potential to the grid for each similar halfcycle of alternating current applied to the tube to fire the tube, anddelaying during the half cycles the time at which the firing potentialis applied as the resistance of the load circuit decreases from themaximum to the substantially zero value in accordance with the increasein current fiow in the load circuit from the substantially constantvalue.

12. The method of maintaining substantially constant the flow of directcurrent through a grid controlled rectifying tube from an alternatingcurrent source to a load'circuit the resistance of which is variablefrom a maximum value to a substantially zero value, which comprisesapplying alternating current firing potential to the grid of the samefrequency as applied to the anode of the tube to cause unidirectionalcurrent to flow in the load circuit, and altering the phase relationbetween the potential applied to the grid and those applied to the anodeas the resistance of the load circuit changes between said maximum andsaid substantially zero values in accordance with the change in directcurrent flow in the load circuit from the substantially constant flow tomaintain the flow substantially constant.

13. In an electrical system of distribution of the class wherein analternating current source energizes a work circuit, the combinationwith said system of a grid controlled arc rectifying tube connectedbetween said source and said work circuit, means for exciting the gridof said tube from said source; means for controlling the phase of thegrid excitation comprising a resistance element. a reactance element anda circuit controlled by adlustment of one of said elements with respectto the other, one of said elements being adjustable; and a currentsensitive device connected to be responsive to the current in the workcircuit, said current sensitive device having a movable member, saidmovable member and said adjustable element being coupled mechanically topermit the adjustable element to be shifted by said movable member toshift the phase of the excitation applied to said grid to limit thecurrent flow in said work circuit to a predetermined value regardless ofthe change in resistance thereof between said maximum and said zeroresistance values.

14. In an electrical system of distribution of the class wherein analternating current source energizes a load circuit including devicessuch as series incandescent lamps, glow and are discharges requiringsubstantially constant current for circuit stability, the combinationwith said system of an anode transformer having a primary winding forconnection to said source and a secondary winding with a mid-tap forconnection to one terminal of said load circuit; a pair of gridcontrolled arc rectifiers, each having an anode, a cathode, and acontrol grid; the anodes being individually connected to the terminalsof said secondary winding and the cathodes being commonly connected tothe other terminal of said load circuit; a current sensitive deviceconnected in series circuit relation with said load circuit and disposedto be responsive to current flow variation therein, a phase splitcircuit comprising an inductance element and a resistance element, oneof which is variable to shift the phase angle of the grid voltage withrespect to the anode voltage of said rectifiers by 180 degrees, saidelements being connected in series circuit relation for energization tosaid current source; a grid transformer having a primary windingconnected between the common connection between said inductance elementand resistance element and a neutrol point on said source to which saidphase split circuit is connected and a secondary winding having amid-tap, the terminals of said last named secondary winding beingconnected to the control grids of said rectifiers' and the mid-tapthereof being connected to said cathodes; and operating meansinterconnecting said current sensitive device and the variable elementof said phase split circuit.

15. In a regulating system for devices such as series incandescentlamps, glow and arc discharges requiring a substantially constant directcurrent for circuit stability, in combination, an anode transformerhaving a primary winding connected to an alternating current supply anda secondary winding, a pair of grid controlled arc rectifiers connectedto said secondary winding for supplying direct current to the loadcircuit, a current sensitive device sensitive to instantaneous currentflow variations, said current sensitive device being connected in seriescircuit relation with theload circuit, a phase split circuit energizedfrom said alternating current supply for energizing the grids of saidare rectifiers, said phase split circuit comprising a network ofvariable inductance and resistance, one of which is controlled by saidcurrent sensitive device to vary the phase of the grid voltage withrespect to the anode voltage to control the current flow through saidrectifiers in such manner as to hold the current constant in the loadcircuit from full load to short circuit or zero load.

HUGH E. YOUNG.

